With a backref, both ends of the relationship are aware of each other -- if you
append object b1 to the collection a.b, then a backref b1.a will be updated
immediately. If you had two different relationships, you wouldn't see an update
to b1.a until you reload the object.
It seems like it'd be nice if they worked the same -- perhaps there's a good
reason it doesn't.
I think 0.4 has an error check for some cases of mutual relationships (A->B,
B->A). Maybe the check is not in place for M:N relationships?
Barry
----- Original Message ----
From: Bobby Impollonia <[EMAIL PROTECTED]>
To: sqlalchemy@googlegroups.com
Sent: Sunday, May 4, 2008 5:28:56 PM
Subject: [sqlalchemy] Re: Duplication of rows in many-to-many relationship
I thought that defining relation with a backref was just a convenient
shorthand for defining two relations. This makes it sound like are
practical differences between the two techniques. Is this true? What
are the differences?
Also, does having the unique key that you recommend stop SA from
trying to add the duplicate? Or will it try anyway and then get a SQL
exception due to the violated constraint?
I am often doing
if a not in b.as:
b.as.append(a)
and I have been wondering if there is a way to just do:
b.as.append(a)
and have SA automatically check if it was already in collection and
shouldn't be added again.
On Sun, May 4, 2008 at 4:40 PM, Barry Hart <[EMAIL PROTECTED]> wrote:
>
> By chance, in your mappers, are you declaring two relationships instead of
> one relation with a backref?
>
> As a side note, once you straighten this out, you may want to declare the
> composite (a_id, b_id) as a unique key on the relation table.
>
> Barry
>
>
> ----- Original Message ----
> From: Karlo Lozovina <[EMAIL PROTECTED]>
> To: sqlalchemy <sqlalchemy@googlegroups.com>
> Sent: Sunday, May 4, 2008 4:31:55 PM
> Subject: [sqlalchemy] Duplication of rows in many-to-many relationship
>
>
> Let's say I have two classes A and B, and I want instances of both
> classes, to have a list of each other, that is, many-to-many
> relationship. For a shorthand, "a" means instance of A, and "b" is an
> instance of B.
>
> For example: "a.bs" is a list, full of instances of class B.
> Similarly, "b.as" is a list, full of instances of class A. In
> modelling that relationship I use three tables, one for As, one for
> Bs, and one for their relationship. If I only append instances of B to
> some "a.bs", then save all those objects, everything works fine. But
> if I append instances of A and B, both to "a.bs" and "b.as", then
> save, I get double rows in the third table. Is there a way around
> that?
>
> P.S.
> In a very likely case I haven't been completely understood, I'll
> attach some code to demonstrate my point ;).
>
> Thanks all.
>
> ________________________________
> Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it
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>
> >
>
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